Here is a paragraph from the linked article by Automattic. It gives at least a partial answer to your question.

We receive hundreds of DMCA notices and try our best to review, identify and push back on those we see as abusive. Our users have the right to challenge a DMCA complaint too, but doing so requires them to identify themselves and fill out a legally required form saying that they submit to being sued for copyright infringement in a place that may be far away. If they don’t, their content is taken down and could stay down forever. This tradeoff doesn’t work for the many anonymous bloggers that we host on WordPress.com, who speak out on sensitive issues like corporate or government corruption.